1. Field of the Invention
The present invention relates to a charging device used in devices, such as laser beam printers, copiers, facsimile devices, or the like, which form images using an electrophotographic method.
2. Description of the Related Art
In an image forming apparatus using an electrophotographic method, in order to charge a surface of a photoconductive drum uniformly, a colotron charging device or a scolotron charging device using corona discharge is generally used.
This is one of the typical charging methods where a photoconductive drum is made to be at an uniform potential by applying high voltage to fine wires to generate the corona discharge.
The fine wires are used for the colotron charging device or the scolotron charging device. However, they are easy to break, so that a replacement of wires is very troublesome. Also, ozone may be evolved when generating the corona discharge, so that such kinds of charging devices may have many other problems.
Therefore, a charging method which is maintained easily and does not evolve ozone, has been recently used. The charging method uses a charging device that includes a charging roller is well-known as a charging device for. Such a charging device will be described while referring to FIGS. 8A and 8B.
As shown in FIG. 8A, a charging roller 40 comprises a semiconductive layer 43 which is provided around a base 41 formed from aluminum and a shaft 42. A power supply 45 is connected to the shaft 42 to apply the necessary voltage for charging. The semiconductive layer 43 is formed by performing a doping or a coating on the base 41 or covering a heat shrinkable tube. On both ends of the charging roller 40, space holder members 44 formed from polyacetal are provided. Each space holder member 44 has a disk shape and a hole provided its center, and the shaft 42 is placed in the holes.
A diameter of the space holder member 44 is slightly larger than that of the charging roller 40 structured with the base 41 and the semiconductive layer 43. Therefore, as shown in FIG. 8B, only the space holder members 44 contact with a photoconductive drum 46, so that a slight narrow discharge space d can be obtained between the photoconductive drum 46 and the semiconductive layer 43. When the voltage is applied to the shaft 42 from the power supply 45, the photoconductive drum 46 is charged uniformly at the predetermined charging potential.
According to such charging method, the space d between the photoconductive drum 46 and the semiconductive layer 43 can be extremely narrow. Therefore, the charging can be performed using low impressed voltage and the evolvement of ozone can be extremely lessened.
According to the conventional charging method, the current flows from the shaft 42 of the charging roller 40 to the semiconductive layer 43 via the base 41, so that it is impossible to obtain a sufficient resistance for charging with stability. The insufficient resistance causes charging defects, such as irregular charging, and reduction of image quality.
One aspect of the invention is to provide a charging device capable of charging with stability. The charging device is so disposed as to leave an extremely narrow space between its surface and a photosensitive member, and includes a charging member for charging the surface of the photosensitive member and a power supply member for supplying a current to the charging member, so disposed as to contact with the surface of the charging member. The power supply member may also have conductivity and elasticity.
The charging device may further comprise a press member for pressing the power supply member toward the surface of the charging member, and a nip width of a portion where the power supply member and the charging member contact each other may be 0.1 mm or more.
A cleaning member for cleaning the surface of the charging member can be so provided as to contact with the surface of the charging member. Then, toner and sheet particles can be removed from the surface of the charging member, so that the photoconductive drum can be charged uniformly.
A cleaning member may also be so provided as to contact with the surface of the power supply member in order to clean the surface of the power supply member.
Further, as the charging member, a roller-shaped member comprising a shaft portion, a base which is made from insulation material and provided around the shaft portion, a charging portion which is made from semiconductive material and provided around the base, and collars which are provided on the both ends of the shaft portion and have a slightly larger diameter than a diameter united the shaft portion, the base, an the charging portion, can be provided.
Still further, as the power supply member, a roller-shaped member comprising a shaft portion and a power supply portion which have conductivity and elasticity and provided around the shaft portion can be provided. A power supply may be connected to the shaft portion.
Moreover, for cleaning the surface of the power supply member, brushes having conductivity and elasticity can be provided on the surface of the power supply member.
Furthermore, a cleaning member for cleaning a cleaning roller may be so provided as to contact with the brushes of the power supply member and the cleaning roller made from conductive material. A power supply capable of applying positive and negative potentials is connected to the cleaning roller, so that positively and negatively charged toner and sheet particles can be removed from the brushes, and the photoconductive drum can be charged uniformly.